Pressure converter

ABSTRACT

A pressure converter insusceptible to the error of hydraulic pressure introducing position or the interference of installation depth of water and producing a signal corresponding to the effective soil pressure accurately while realizing cost reduction. Since filters are provided in an air gap defined by two rigid plates receiving the soil pressure, a hydraulic pressure is applied but sand pressure is not applied. A load gauge insensitive to the hydraulic pressure detects only the effective soil pressure. Since a signal cable can be passed through the air gap, a plurality of pressure converters can be coupled efficiently and since the signal cable can be inserted internally, a pressure distributed in the depth direction can also be measured accurately.

BACKGROUND OF THE INVENTION

The present invention relates to a pressure converter and morespecifically to a pressure converter for detecting soil pressure (hereafter, referred to as “effective earth pressure”) which eliminates porewater pressure in the ground, such as at the bottom of dams, in riverbeds, sea beds and sediment slopes, and also for detecting a microfluctuation of the pore water pressure (hereafter, referred to as“dynamic pore water pressure”) among soil particles forming the groundand then outputting such data as transmission signals.

Naturally, as the ground at the bottom of a dam, river bed, sea bed orsediment slope is repeatedly accumulated and eroded, sediment gravel,dirt and sand determines geological features and influences structuresconstructed on the ground, which makes it necessary to examine soilpressure (hereafter, referred to as “earth pressure”) in the groundbeforehand. The earth pressure generally includes soil skeleton pressureof soil particles forming the ground and pore water pressure among thesoil particles. This pressure is generally called “total earthpressure.” In addition, the pore water pressure is the pressure of airand water mixture.

Furthermore, the earth pressure, which excludes pore water pressure fromthe total earth pressure is called “effective earth pressure.”Therefore, it is possible to determine whether the ground is strongenough for constructing underground structures or whether the slope ofthe ground is stable by measuring the effective earth pressure or thepore water pressure in the ground. It is also becoming possible todetermine whether the surface of sediment dirt and sand has risen orfallen.

A conventional example for conducting such measurement is an electricalpressure converter which the present inventor et al. have proposed inU.S. Pat. No. 2,696,099.

FIG. 12 is a cross-sectional view showing the configuration of aneffective earth pressure gauge according to the conventional example(hereafter, referred to as “first conventional example”).

In FIG. 12, at the center of the front and rear surfaces of the strainstarting, portion 81, pressure receiving rods 83, 83 are united as onestructure or integrally fixed so that the rods coaxially protrude fromthe main body 82. A sword-guard shaped pressure receiving plate 84, 84is provided at each top end of the pressure receiving rod 83, 83.

An expandable bellows 85, 85 is mounted between each pressure receivingplate 84, 84 and the main body 82 so that the bellows 85, 85 seals theopening to prevent moisture from sinking into the above-mentioned strainstarting portion 81 side. At a central portion and a peripheral edge ofboth front and rear surfaces of the strain starting portion 81, aplurality of strain gauges 86 are attached to function as elements whichconvert the amount of deformation of the strain starting portion 81 intoan electrical signal, thereby forming the Wheatstone Bridge, not shownin the drawing.

On the outer peripheral edge side of one (the upper one in the drawing)of the above-mentioned pressure receiving plates 84 and bellows 85, afilter 93 is mounted to the main body 82 by a screw-in means or the likeso that a hydraulic pressure chamber 88 is defined by the surroundingfilter. Furthermore, on the outer peripheral edge side of the otherpressure receiving plate 84 and bellows, a portion of the main body 82is projected like a sword-guard, and a thin pressure-receiving diaphragm94 is installed on the cross-section of the main body to form adetecting portion 95 which faces the pressure receiving plate 94 withmicro pores interposed. The detecting portion 95 is filled with a liquid96, such as hydraulic oil or the like.

AAn earth pressure gauge, configured as stated above, is designed sothat when it is buried in the ground, the surface of the pressurereceiving diaphragm 94 receives overalltotal earth pressure includingsoil skeleton pressure of soil particles and pore water pressure. On theother hand, the filter 93 prevents soil particles from entering butallows only pore water to enter so that the hydraulic pressure chamber88 receives pore water pressure.

Therefore, between soil skeleton pressure and pore water pressureapplied to the pressure receiving diaphragm 94, the pore water pressureis cancelled by the pore water pressure on the filter 93 side, andtherefore, only soil skeleton pressure (this is called “effective earthpressure”) is applied to the strain starting portion 81 via the pressurereceiving plate 84 and the pressure receiving rod 83. Accordingly, anelectrical signal corresponding to the effective earth pressure can beobtained by the strain gauge 86. This earth pressure measurement detectsthe magnitude of the earth pressure in the ground, which makes itpossible to determine the strength of the ground before structures areconstructed.

FIG. 13 is a cross-sectional view showing the configuration of a dynamicpore water pressure gauge disclosed in U.S. Pat. No. 2,696,099(hereafter, referred to as “second conventional example”).

In FIG. 12, on the outer periphery side of the lower pressure receivingplate 84 and bellows 85, a portion of the main body 82 is projected likea sword-guard, and a thin pressure-receiving diaphragm 94 is installedon the cross-section to form a detecting portion 95 which faces thepressure receiving plate 94 with micro pores interposed. However, inFIG. 13, instead of providing the detecting portion 95, a filter 89,having the attenuation characteristic different from the upper filter90, is mounted to the main body 82 by a screw-in means or the like sothat a hydraulic pressure chamber 87 is defined as the result of thefilter surrounding the pressure receiving plate 84 and the bellows 85.

As shown in the characteristics diagram in FIG. 14 showing therelationship between the fluctuation frequency (Hz) of the measuredpressure and the attenuation factor of the filter [Pb/Po], one filter 89uses a rough-mesh filter with the A-characteristic curve which does notattenuate pressure waves until a high frequency is reached, and theother filter 90 uses a filter with the B-characteristic curve whichattenuates pressure waves at a lower frequency compared to theabove-mentioned filter 89. Furthermore, concerning symbols indicatingthe attenuation factor of the above-mentioned filters, Po denotes aninput pressure value and Pb denotes an output signal pressure value.

A dynamic pore water pressure gauge, configured as stated above, isburied in the ground of the seabed, for example. Hydraulic pressurecaused by water depth, that is, static hydraulic pressure (hereafter,referred to as “hydrostatic pressure”, is applied through the filters89, 90 without generating a pressure difference between the hydraulicpressure chambers 87, 88 even if attenuation characteristics of thefilters 89, 90 are different; and pare water pressure is uniformlyapplied to the pressure receiving plates 84, 84. As a result, the strainstarting portion 81 is not deformed and the strain gauge 86 does notgenerate any electrical outputs, and therefore, hydrostatic pressure iscancelled.

However, if dynamic pore water pressure occurs due to the wave having aspecific period in the pore water within the ground, the difference inthe attenuation characteristics of the filters 89, 90 causes a pressuredifference between two hydraulic pressure chambers 87, 88.

That is, as FIG. 14 shows, because dynamic pore water pressure in thehydraulic pressure chamber 87 side which has a filter 89 having theA-characteristic curve becomes high, the pressure deforms the strainstarting portion 81 via a pressure receiving plate 84, and the amount ofdeformation of the strain starting portion 81 is detected as anelectrical resistance value by a strain gauge 86. Accordingly, it ispossible to detect an electrical signal corresponding to the dynamicpore water pressure from the output end of the bridge formed by thestrain gauge.

As a consequence, regardless of the magnitude of the hydraulic pressurecaused by water depth or earth pressure, it is possible to measure amicro fluctuation of the dynamic pore water pressure caused by anearthquake or other movements of the earth's crust. Furthermore, thedynamic pore water pressure data indicates the strength of the ground,which is an excellent indication used for the design, construction andsafety management of the structures to be constructed.

The effective earth pressure gauge according to the above firstconventional example and the dynamic pore water pressure gauge accordingto the above second conventional example enable the measurement of theeffective earth pressure and the dynamic pore water pressure. However,each of those conventional examples uses two bellows 85, 85 having thesame spring constant and applies the configuration of a differentialpressure gauge in which the strain starting portion 81 located insidethose bellows 85, 85 is deformed due to a liquid pressure generatedoutside the bellows 85.

In the configuration that uses two bellows 85, 85, as FIGS. 12 and 13show, there is a head difference between the height from thedifferential pressure center position to the bellows' pressure loadposition A and the height from the differential pressure center positionto the bellows' pressure load position B, which affects the measuredpressure. Therefore, this becomes a problem when generated effectivestress has to be measured highly accurately with an error of less than1-CM head.

If the device shown in FIGS. 12 and 13 is rotated 90-degrees andinstalled, the head difference can be eliminated. In that case, however,when effective earth pressure is measured, symmetry in the verticalearth pressure direction is deformed, and when dynamic pore waterpressure is a measured, the transverse rectangle shape makes itdifficult to insert and install the device into the ground, that is,generally through a small-diameter bore hole.

Moreover, in fact, since it is difficult to obtain two bellows whichhave quite the same spring constant, bellows which have approximatelysimilar spring constant may be used. Due to the difference between thosespring constants, an interference output which has nothing to do withthe hydrostatic pressure caused by the depth of installation is appliedto the bellows as an interference output. In an effective earth pressuregauge, as FIG. 12 shows, earth pressure is applied from the pressurereceiving diaphragm 94 to one bellows 85 via the pressure of the liquid96 that fills in the diaphragm chamber; and in some cases, the effectcannot be ignored.

Recently, a plurality of effective earth pressure gauges and dynamicpore water pressure gauges are coupled to measure the effective earthpressure and the dynamic pore water pressure distributed in the depthdirection.

However, if a plurality of bellows-type pressure converters according tothe above-mentioned first and second conventional examples is arecoupled in the multi-stage arrangement, the problems described belowarise.

That is, if a first pressure converter 110, a second pressure converter120, a third pressure converter 130 and so on are sequentially connectedin the multi-stage arrangement, as shown in FIG. 15, via connectingpipes 114, 124, 134, signal cables 111, 121, 131 must be extended to theother sides along the outer surface of the pressure converters 110, 120,130.

In addition, to protect each signal cable 111, 121, 131, a protectivemember 113, 123, 133, and a protective pipe 112, 122, 132 that isconnected to the protective member must be installed so that the signalcable 11, 121, 131 can pass through the protective member and theprotective pipe.

As FIG. 15 shows, the signal cable 111, 121, 131 extends from theconnecting pipe 114, 124, 134, passes through the protective members113, 123, 133, and then extends along the outer circumferential surfacesof the cylindrical main bodies of the first through third pressureconverters 110 through 130 and the filters 115, 125, 135 while beingcovered by the protective pipe 112, 122, 132. Accordingly, theconfiguration around the cable becomes large, which may disturb theobserved soil pressure or the signal cable may be exposed to anenvironment where it is easily abraded.

BRIEF SUMMARY OF THE INVENTION

To solve the above-mentioned problems, the first objective of thepresent invention is to provide a pressure converter, which can detecteffective earth pressure by means of a direct differential force.

The second objective of the present invention is to provide a pressureconverter, which can detect effective earth pressure, thatwhich is notinterfered by hydrostatic pressure caused by the depth of installation.

Furthermore, the third objective of the present invention is to providea pressure converter, which is easily installed horizontally and easilyinserted vertically.

The fourth objective of the present invention is to provide a pressureconverter, which can easily detect pressure distributed in thelongitudinal direction.

Moreover, the fifth objective of the present invention is to provide apressure converter, which can detect dynamic pore water pressurethatwhich is not interfered by hydrostatic pressure caused by the depthof installation.

To achieve the above-mentioned first through third objectives, theinvention is configured such that a load gauge, which does not causeoutput fluctuation even if air pressure and hydraulic pressure aredirected via a filter and applied between two rigid plates, is installedin an air gap defined by two rigid plates which receive earth pressure,and said load gauge detects a signal corresponding to the effectiveearth pressure.

To achieve the above-mentioned first through third objectives, theinvention is configured such that a thin-walled elastic portion isdisposed at an end portion of at least one of said two rigid plates sothat a load in proportion to earth pressure can be transmitted to saidload gauge without disturbing the deformation by earth pressure.

To achieve the above-mentioned first through third objectives, theinvention is configured such that the shape of the pressure receivingsurface of said rigid plate is circular, a thin-walled elastic portionis disposed in the vicinity of the peripheral edge of the oppositesurface from the pressure receiving surface of at least one rigid plate,said two rigid plates are disposed so that their opposite surfaces fromthe pressure receiving surfaces inwardly face each other, said loadgauge is interposed between the central portions of said two rigidplates, and a support and a filter are interposed between the peripheraledge portions of said two rigid plates.

To achieve the above-mentioned first through third objectives, theinvention is configured such that the shape of the pressure receivingsurface of said rigid plate is rectangle, a thin-walled elastic portionis disposed in the vicinity of the longitudinal end portion of theopposite surface from the pressure receiving surface of at least onesaid rigid plate, the opposite sides of the pressure receiving surfacesinwardly face each other, said load gauge is interposed between thecentral portions of said two rigid plates, a support is interposedbetween the longitudinal end portions of said two rigid plates, and afilter is interposed between the transverse end portions of said tworigid plates.

To achieve the above-mentioned first through third objectives, theinvention is configured such that the shape of the transversecross-section of the pressure receiving surface of said rigid plate isgenerally semicircular, a thin-walled elastic portion is disposed in thevicinity of the longitudinal end portion of at least one rigid plate,opposite sides of the pressure receiving surfaces inwardly face eachother, said load gauge is interposed between the central portions ofsaid two rigid plates, a support is interposed between the longitudinalend portions of said two rigid plates, and a filter is interposedbetween the transverse end portions of said two rigid plates.

To achieve the above-mentioned first through fourth objectives, theinvention is configured such that said two rigid plates are coupled inthe longitudinal direction in a multi-stage arrangement, and an outputsignal cable of said load gauge successively passes through an air gapdefined as the result of said load gauge being interposed between saidtwo rigid plates.

To achieve the above-mentioned first through third objectives, theinvention is configured such that a thin-walled elastic portion isdisposed at an end portion of at least one of two rigid plates whichreceive earth pressure, an optical-fiber strain gauge or strain gauge isattached to the opposite surface side from the pressure receivingsurface of said thin-walled elastic portion, the opposite sides of thepressure receiving surfaces inwardly face each other, a support and afilter are interposed between the end portions of said two rigid plates,and said optical-fiber strain gauge or strain gauge, which does notcause output fluctuation even if air pressure and hydraulic pressure aredirected via said filter and applied between said two rigid plates,detects a signal corresponding to the effective earth pressure.

To achieve the above-mentioned fifth objective, the invention isconfigured such that a filter having slow responsiveness to hydraulicpressure is interposed between the end portions of said two rigid plateswhich receive hydraulic pressure, a load gauge is interposed between thecentral portions of said two rigid plates, a filter having fastresponsiveness to hydraulic pressure is disposed so that it surroundssaid two rigid plates, and said load gauge detects a signalcorresponding to the dynamic pore water pressure.

To achieve the above-mentioned fifth objective, the invention isconfigured such that a thin-walled elastic portion is disposed at an endportion of at least one of two rigid plates which receive earthpressure, an optical-fiber strain gauge or strain gauge is attached tothe opposite surface side from the pressure receiving surface of saidthin-walled elastic portion, the opposite sides of the pressurereceiving surfaces inwardly face each other, a filter having slowresponsiveness to hydraulic pressure is interposed between the endportions of said two rigid plates, a filter having fast responsivenessto hydraulic pressure is disposed so that it surrounds said two rigidplates, and said optical-fiber strain gauge or strain gauge detects asignal corresponding to the dynamic pore water pressure.

To achieve the above-mentioned third and fourth objectives, theinvention further comprises a plurality of supports provided betweenadjacent pressure converters and a coupling plate which extends betweenadjacent supports and is mounted to said supports so as to connect aplurality of pressure converters in a longitudinal column arrangement,thereby distributing loads in the directions other than the measurementdirection to said supports and said coupling plate. This configurationprevents loads other than the earth pressure in the measurementdirection from being applied during the installation, insertion andmeasurement processes.

When a pressure converter, configured as stated above, is turned in theground, hydrostatic pressure caused by water depth, that is, statichydraulic pressure is directed via the filter to an air gap defined bytwo rigid plates which receive the earth pressure. Since a pressuredifference does not occur at a load point, said hydrostatic pressure iscancelled.

On the other hand, soil skeleton pressure in the ground, that is, aneffective earth pressure load becomes a force which pushes agains thepressure receiving surfaces of a pair of rigid plates and is applied tothe load gauge. Consequently, a signal corresponding to the effectiveearth pressure is detected by the load gauge.

Although the hydrostatic pressure in the ground is cancelled as statedabove if the pore water pressure caused by pore water in the ground hasthe waveform with a cycle greater than the prescribed cycle resulting influctuations, as shown in the present invention, a pressure differenceoccurs between the inside and outside of an air gap due to thedifference of attenuation characteristics between a filter having slowresponsiveness to hydraulic pressure which is interposed between the endportions of two rigid plates which receive a hydraulic pressure and afilter having fast responsiveness which is provided to surround the tworigid plates. This pressure difference deforms a load gauge interposedbetween the two rigid plates, and the amount of deformation is convertedinto an electrical signal by an element such as a strain gauge or anoptical-fiber strain gauge, thereby an electrical signal correspondingto the pore water pressure can be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

Accompanying the specification are figures which assist in illustratingthe embodiments of the invention, in which:

FIG. 1 is a cross-sectional view showing the configuration of a pressureconverter for detecting effective earth pressure, according to a firstembodiment of the present invention, wherein rigid plates are discoid;

FIG. 2 is a plan view of the pressure converter shown in FIG. 1;

FIG. 3 is a cross-sectional view showing the configuration of a pressureconverter for detecting vertical effective earth pressure, according toa second embodiment of the present invention, wherein the peripheraledge of the rigid plates is not fixed;

FIG. 4 is a semi-cross-sectional view showing the configuration of apressure converter unit according to third and fourth embodiments of thepresent invention, which can detect a plurality of effective earthpressures;

FIG. 5 is a front view of the pressure converter shown in FIG. 4;

FIG. 6 is a perspective view showing the configuration of the pressureconverter shown in FIG. 4;

FIG. 7 is a cross-sectional view viewed in the direction of the B—B lineshown in FIG. 4;

FIG. 8 is a cross-sectional view viewed in the direction of the C—C lineshown in FIG. 4;

FIG. 9 is a partially broken side view showing the configuration of apressure converter according to a fifth embodiment of the presentinvention, which can detect dynamic pore water pressure;

FIG. 10 is a cross-sectional view viewed in the direction of the D—Dline shown in FIG. 9;

FIG. 11 is a diagram showing the principle of non-interference ofhydraulic pressure in a load gauge used for first through fourthembodiments of the present invention;

FIG. 12 is a cross-sectional block diagram of an effective earthpressure gauge according to a first conventional example;

FIG. 13 is a cross-sectional block diagram of a dynamic pore waterpressure gauge according to a second conventional example;

FIG. 14 is a response diagram of a dynamic pore water pressure gaugeaccording to a second conventional example; and

FIG. 15 is a side view showing the configuration when a plurality ofeffective earth pressure gauges according to other conventional examplesis are connected.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, embodiments of the present invention will be described indetail with reference to the drawings.

FIGS. 1 and 2 show the configuration of a pressure converter, accordingto a first embodiment of the present invention, which can detecteffective earth pressure. FIG. 1 is a cross-sectional view viewed in thedirection of the A—A line shown in FIG. 2, and FIG. 2 is a plan view.

In FIGS. 1 and 2, two circular rigid plates 11, 12 are thick rigidplates made of stainless steel or the like. The rigid plate looks like adisc in a plan view and the pressure receiving surface is made flat. Onthe other hand, at the central portion of the opposite surface from thepressure receiving surface, a shallow concave portion is formed toinstall a load gauge 41, and a groove 11 b, 12 b whose cross-sectionalform is trapezoid is annularly cut out in the vicinity of the peripheraledge, resulting in forming a thin-walled elastic portion 11 a, 12 a.

Thus, each of two circular rigid plates 11, 12 consists of a thick rigidportion from the central portion toward the vicinity of the peripheraledge, a thin-walled elastic portion (flexible portion) in the vicinityof the peripheral edge, and a thick mounting portion in the peripheraledge area.

In this embodiment, two circular rigid plates 11, 12 are designed as thesame shape or symmetrical shape. Their opposite surfaces from thepressure receiving surfaces, that is, the surfaces on which the grooves11 b, 12 b are formed inwardly face each other. And, a load gauge 41 isdisposed in the shallow concave portion created at the central portionsof the two circular rigid plates 11, 12, and three supports 21 areinterposed at equiangular (120 degrees) intervals between theirperipheral edge portions. And then, the upper and lower load introducingportions (fulcrum) of the load gauge 41 are mounted to the circularrigid plates 11, 12 by a screw-in means or the like. At this point, asignal cable 51 for directing signals from the load gauge 41 isexternally pulled out through an air gap 10 defined by the two circularrigid plates 11, 12.

After that, two filters 31, 32 are annularly interposed between the twocircular rigid plates 11, 12 in the vicinity of the peripheral edgeswith a constant interval in the radius direction provided between thosetwo filters. This configuration allows air and water to enter an air gap10, but blocks large particles of dirt and sand, thereby preventing thedeformation of the rigid plates 11, 12 due to dirt and sand that hasentered the air gap and also preventing the elastic deformation of thethin-walled elastic portions 11 a, 12 a due to earth pressure.

As FIG. 11 shows, the load gauge 41 used herein detects the load havingtwo fulcrums: two load introducing portions 41 a, 41 b. This load gaugein principle produces no output when liquid pressure around the loadgauge 41 changes the pressure, that is, a pressure interference error issignificantly small.

When earth pressure P acts on the pressure converter 1 having such aload gauge 41, as shown in FIG. 1, pore water pressure Pw included intotal earth pressure P is directed into the air gap 10 through thefilters 3, 32; however, soil skeleton pressure Ps cannot enter the airgap 10. Therefore, load F, which is defined by the following equation,acts on the load gauge 41.F=(Ps+Pw)·A−Pw·A=Ps·A  (1)

Herein, P=Ps+Pw, P: total earth pressure, Ps: soil skeleton pressure(effective earth pressure)

Pw: pore pressure, A: earth pressure gauge's effective loading area

To facilitate understanding of the principle, the thin-walled elasticportion 11 a of the two rigid plates 11, 12 is used to explain theprinciple. The thin-walled elastic portion 11 a functions as a spring,which is elastically deformed in proportion to the earth pressure aswell as the load gauge 41. Therefore, assuming that the spring constantof the thin-walled elastic portion 11 a is represented by k1 and thespring constant of the load gauge 41 is represented by k2, springconstant k at the central portion of the effective earth pressure gaugecan be defined by equation (2).k=k1+k2  (2)

In the light of the equation (2), even if a plurality of load gauges 41,four load gauges for example, are installed between the two circularrigid plates 11, 12 at equiangular intervals according to the on-sitesituation, the operation principle does not change. Furthermore, as seenin a second embodiment shown in FIG. 3, it is possible to use elasticsupport of the load gauge 41 only instead of providing a thin-walledelastic portion 11 a and a support 21.

In this configuration, all that the filter 33, shown in FIG. 3, has todo is to prevent sand intrusion from the air gap 10; therefore, simplesynthetic fiber or sponge can be utilized.

Although the above explanation uses circular rigid plates 11, 12, theirpressure receiving surfaces can be rectangle or semicircular as thirdand fourth embodiments show. In that case, it is possible to measuredistribution pressure by connecting a plurality of effective earthpressure gauges in the multi-stage arrangement.

Next, with reference to FIGS. 4, 5, 6 and 7, a third embodiment, whichuses a plurality of pressure converters having rectangle rigid plates,will be explained. FIG. 4 is a side cross-sectional view, FIG. 5 is anenlarged plan view, FIG. 6 is a perspective view, FIG. 7 is across-sectional view viewed in the direction of the B—B line shown inFIG. 5, and FIG. 8 is a cross-sectional view viewed in the direction ofthe C—C line shown in FIG. 5.

An insertion head 71 is mounted to the base end (lower end in FIG. 5) ofthe support 23 to reduce resistance as the device is inserted into theground on site.

In FIG. 4, two rectangle rigid plates 13, 14 are mounted to the support23 with a set screw at a clamping portion located outside of one site ofthin-walled elastic portions 13 a, 14 a, and they are also mounted tothe support 24 with a set screw at a clamping portion located outside ofthe other thin-walled elastic portions 13 b, 14 b.

However, before the above clamping work is conducted, a load gauge 41must be installed in the air gap 10 defined by the two rectangle rigidplates 13, 14 according to the above-mentioned procedures.

Instead of using a load gauge 41, optical-fiber strain gauges or straingauges (including semiconductor gauge) can be directly installed at fourthin-walled elastic portions 13 a, 13 aa, 13 b, 13 bb so that signalsaccording to the load can be detected.

Filters 34, 35 are mounted to both of the open end portions of the airgap 10 defined by two rectangle rigid plates 13, 14 facing each other,with a small clearance provided so that the filters do not hinder themovements of the rectangle rigid plates 13, 14.

That is, one end of the filter 34, 35 is mounted to a support 23 and theother end is mounted to a coupling support 24 with set screws. One endof a lengthy coupling plate 61 is mounted to the support 23 and theother end is mounted to the coupling support 24 with set screws. Thiscoupling plate 61 connects a plurality of pressure converters one by oneand also distributes torque caused by the pressure converters axialdirection (longitudinal direction) tension or the compression directionforce or torsion so that only the measurement direction earth pressureis applied to the pressure receiving surfaces of the two rigid plates13, 14.

As FIGS. 4, 5 and 6 show, coupling portions 24 b, 24 bb of the support24 are connected with set screws in the vicinity of the thin-walledelastic portions 13 b, 13 bb of the rectangle rigid plate 13 of thepressure converter 1 and in the vicinity of the thin-walled elasticportion 14 b of the rectangle rigid plate 14.

On the other hand, the other coupling portions 24 a, 24 aa of thesupport 24 are coupled with set screws in the vicinity of thethin-walled elastic portions 15 a, 15 aa of the rectangle rigid plate 15of the pressure converter 2 located on the following stage and in thevicinity of the thin-walled elastic portion 16 a of the rectangle rigidplate 16.

In this way, a plurality of pressure converters are connected one by oneto form a multi-stage pressure converter which can detect pressuredistributed in the axial direction, that is, effective earth pressure.

A signal cable 51 for externally directing signals outputted from theload converter 41 passes through the air gap 10, defined by tworectangle rigid plates 13, 14, and then passes through a through-hole 24c drilled in the coupling support 24 that couples the pressureconverters 1, 2 (FIG. 8), and subsequently in the same manner, thesignal cable can pass through the air gap: 0 and the through-hole 24 c.Consequently, the signal cable does not come out from the pressureconverter as shown in FIG. 15.

Therefore, the signal cable does not disturb the dirt and sand aroundthe pressure converter nor is it exposed to an environment where it iseasily abraded. Accordingly, it is possible to obtain accuratemeasurements and increase durability, reliability and usability.

Filters 34, 35 in this third embodiment use perforated metal plates orthe like so that they withstand abrasion caused when it is inserted intothe ground and also help lower production costs.

Next, a fourth embodiment in which two rigid plates have differentfunctions and shape will be explained.

As chain double-dashed lines (virtual lines) show in FIGS. 4 and 7, theshape of the transverse cross-section of the pressure receiving surfaceof the rigid plate 13′, 14′ is generally semicircular, and thin-walledelastic portions 13 a, 13 b, 14 a, 14 b are provided in the vicinity ofthe peripheral edge of the two rigid plates 13′, 14′. The oppositesurfaces from the pressure receiving surfaces inwardly face each otherwith a load gauge 41 interposed between the central portions of the tworigid plates 13′, 14′. Furthermore, a support 23 and a coupling support24 are interposed between the longitudinal end portions of the two rigidplates 13′, 14′, and filters 34, 35 are interposed between thetransverse end portions.

This configuration allows the surface for sensing pressure to almostlook like a wall surface of a circular bore hole. Therefore, when theearth pressure of relatively hard ground is measured, since the pressurereceiving surface and the bore hole wall surface look alike, soilskeleton pressure is uniformly applied to the entire pressure receivingsurface, resulting in detecting effective earth pressure that is moreadaptable to the practical situation.

Moreover, the two rigid plates 13′, 14′ may be semi-cylindrical as longas the surface for sensing pressure is semicircular. It is also possibleto bend a thick plate to form a semicircular plate and integrally fix itto each surface of the flat rigid plate 13, 14, as shown in FIG. 7, bymeans of welding or the like.

Between those two configurations, the latter is more advantageousbecause the device weighs less.

Next, FIGS. 9 and 10 show a pressure converter according of a fifthembodiment of the present invention.

FIG. 9 is a partially broken front view, and FIG. 10 is across-sectional view viewed in the direction of the D—D line shown inFIG, 9.

The basic configuration of the pressure converter according to a fifthembodiment is the same as the pressure converter according to a thirdembodiment shown in FIGS. 4 through 7. In this embodiment, first filtershaving slow responsiveness to hydraulic pressure are mounted to open endportions of two rigid plates 13, 14, and another cylindrical filter 35with fast responsiveness to hydraulic pressure is disposed such that thefilter surrounds the two rigid plates; and a load gauge 41 installedbetween the two rigid plates detects signals corresponding to thedynamic pore water pressure.

The fifth embodiment will be explained in detail by referring to FIGS. 9and 10.

In the pressure converter 1, in the same manner as the pressureconverter shown in FIGS. 4 through 7, an insertion head 72 is clamped atone end of the support 23, and a pair of rigid plates 13, 14 extend fromthe support 23 to the coupling support 24 and are clamped. A load gauge41 is installed between a pair of rigid plates 13, 14.

Furthermore, first filters 34, 35 perforated with micro holes, in otherwords, having low responsiveness to hydraulic pressure are installedbetween the support 23 and the coupling support 24 and their endportions are clamped with set screws.

Moreover, altogether four coupling plates 61 are installed on the sidesof those filters 34, 35 with their both ends mounted to the support 23and the coupling support 24.

A second filter 36 perforated with large holes, in other words, havingfast responsiveness to hydraulic pressure is installed such that thefilter surrounds the outer circumferential surface of the pressureconverter 1 which is configured as stated above.

Particularly, the second filter 36 uses a metal cylinder, which ispartially perforated so that it can withstand abrasion when it isinserted into the ground.

In the pressure converter configured as stated above, as FIG. 10 shows,two air gaps are created: an internal air gap 10 defined by two rigidplates 13, 14 and two first filters 34, 35; and an intermediate air gap20 defined by the circumferential surfaces of the two rigid plates 13,14 and the cylindrical second filter 36.

Because capacity of the internal air gap 10 is relatively large, if aport to which pressure is applied from the first filter 34, 35 side ismade narrow so as to delay response to the fluctuation of hydraulicpressure, almost no fluctuation which is almost equal to the hydrostaticpressure occurs in the internal air gap 10.

Since a measurement signal outputted from the load gauge 41 is detectedas a difference between a pressure fluctuation in the intermediate airgap 20 and a pressure fluctuation in the air gap 10, a value calculatedby subtracting hydrostatic pressure Pws from pore water pressurefluctuation Pwd, that is, a signal corresponding to the dynamic porewater pressure can be detected.

The cylindrical second filter's hydraulic-pressure introducing portion36 a, which is a major measurement location, functions as a filterhaving fast responsiveness to hydraulic pressure and the other area ofthe filter is used to connect a plurality of main bodies.

Now, with reference to FIGS. 11 a and 11 b, the principle of the loadgauge 41 will be described.

FIG. 11 a is a plan view showing the configuration of the load gauge,and FIG. 11 b is a front view showing the operation of the load gauge.

Strain gauges G1 through G4 are mounted to the bending beam of the loadgauge shown in FIGS. 11 a and 11 b.

When load F is applied to the working points 41 a, 41 b at which theload is directed, gauges G1, G3 are compressed and distorted by theamount εc=εm, and gauges G2, G4 are pulled and distorted by the amountεt=−εm. Therefore, output fluctuation εs of the bridge consisting offour strain gauges G1 through G4 can be expressed by the followingequation:εs=2(εt−εc)≈−4εm  (3)

On the other hand, when hydraulic pressure Pp is applied to the bendingbeam, assuming that the bending beam's Young's modulus is Eb, axialdirection strain εz caused by hydraulic pressure Pp can be expressed bythe formula εz=Pp/Eb, and this value becomes equal to the amount ofstrain in four strain gauges G1 through G4. In this case, outputfluctuation εsz can be expressed by the following equation:εsz=2(εz−εz)≈0  (4)

Furthermore, even if bending strain increased by em due to the influenceof hydraulic pressure, the amount of strain in each strain gauge G1through G4 is equal to the amount in the same manner as the axial strainand denoted by the same symbols. Therefore, the bridge's outputfluctuation εsm can be expressed by the following equation:εsm=2(εm−εm)≈0  (5)

Both equations (4) and (5) indicate that an interference outputinfluenced by ambient pressure does not occur in principle.

In an actual load gauge that uses strain gauges, strain gauges G1through G4 must be electrically insulated. Therefore, in the pressureconverter shown in FIGS. 11 a, 11 b, the strain gauges are attachedinside so that they do not directly come in contact with water orliquid.

Recently, optical-fiber strain gauges have been developed. In one ofrepresentative examples of the optical-fiber strain gauge, a method,which uses transmitted light and applies the transmission factor thatfluctuates according to the flexion of the optical fiber, has been inthe practical use. In the case of the optical fiber that usestransmitted light, one reciprocating motion of the optical fiber enablesa plurality of strain measuring points to be measured. Therefore, bymounting an optical-fiber strain gauge at a bending strain detectingposition of the above-mentioned bending beam's gauges G1 through G4, itis possible to configure a pressure converter, which can accuratelydetect the distribution of the effective earth pressure or dynamic porewater pressure.

This optical-fiber strain gauge does not require electrical insulation.Therefore, it is not necessary to have such a sealing structure asinstalling a strain gauge in a hollow as the device shown in FIGS. 11 a,11 b does, resulting in facilitating installation of the signal line.

In the thin-walled elastic portions 13 a, 13 aa, 13 b, 13 bb shown inFIGS. 4, 5 and 6, axial strain or bending strain caused by pressureinterference does not occur. Therefore, by mounting a transmissibleoptical-fiber strain gauge onto the opposite surface side from thepressure receiving surface sensing pressure of the thin-walled elasticportions 13 a, 13 aa, 13 b, 13 bb instead of providing a load gauge 41,it is possible to detect signals corresponding to the effective earthpressure.

Since a plurality of transmissible optical-fiber strain gauges canmeasure strain by one reciprocating motion of the optical fiber, apressure converter according to the present invention makes the best useof the characteristics to significantly reduce measurement costs.

Moreover, the present invention is not limited to the embodiments whichhave been described and illustrated above and can be embodied in avariety of forms as long as they are not departed from the concept ofthe present invention.

For example, in the above statement, two rigid plates are symmetrical;however, a thin-walled portion or a thin-walled elastic portion may beprovided in the peripheral edge portion of only one rigid plate.

This configuration eliminates the necessity to create a groove and slitto form a thin-walled elastic portion on the other rigid plate,resulting in reduction of production costs.

However, the above-mentioned embodiments, which use a pair ofsymmetrical-shaped rigid plates, are more advantageous because they aremore sensitive and can more accurately detect effective earth pressureor dynamic pore water pressure.

Furthermore, the spring constant of the thin-walled portion orthin-walled elastic portion may be adjusted by creating aletter-U-shaped slit, which can be seen in the thin-walled elasticportions 13 a, 13 aa or thin-walled elastic portions 13 b, 13 bb inFIGS. 5 and 6.

If thin-walled portions are designed to have a small spring constant,resulting in a possibility of being broken due to insufficient strength,a slit can be provided to make thin-walled elastic portions 13 a, 13 aa,13 b, 13 bb narrow but thick, thereby making them sufficiently strong.

Furthermore, it is also possible to process thin-walled elastic portions13 a, 13 aa, 13 b, 13 bb into an accurately machined unit and then clampit at a specified location with set screws.

As is clear from the above statement, according to the presentinvention, it is possible to detect fluctuation of the effective earthpressure by a load gauge which is installed in the air gap defined by afilter that prevents the application of the soil skeleton pressure andtwo rigid plates and does not cause output fluctuation due to theinfluence of hydrostatic pressure. It is also possible to coincide theeffective earth pressure measurement position with the pore waterpressure application position at the time of the measurement.Accordingly, because there is no head difference between the positionand the pressure loading position, it is possible to provide a pressureconverter, which is capable of accurately measuring the effective earthpressure.

According to the present invention, a thin-walled elastic portion isdisposed at an end portion of at least one of two rigid plates so that aload in proportion to earth pressure can be transmitted to the loadgauge without disturbing the deformation by earth pressure. Because theend portions of the two rigid plates can be clamped, this configurationmakes it possible to provide a pressure converter, which facilitatestransportation and installation and also enables stable measurement ofthe effective earth pressure.

According to the present invention, the shape of the pressure receivingsurface of said rigid plate is circular, a thin-walled elastic portionis disposed in the vicinity of the peripheral edge of the oppositesurface from the pressure receiving surface of at least one rigid plate,said two rigid plates are disposed so that their opposite surfaces fromthe pressure receiving surfaces inwardly face each other, said loadgauge is interposed between the central portions of said two rigidplates, and a support and a filter are interposed between the peripheraledge portions of said two rigid plates. Thus, it is possible to providea pressure converter, which is suitable in cases when the ground of themeasurement site is flat and a device to be buried must be circular, andis also versatile, has a simple configuration that requires lowproduction cost and is capable of accurately detecting the effectiveearth pressure.

Furthermore, according to the present invention, the shape of thepressure receiving surfaces of two rigid plates is rectangle, athin-walled elastic portion is disposed in the vicinity of thelongitudinal end portion of the opposite surface from the pressurereceiving surface of at least one rigid plate, the opposite sides of thepressure receiving surfaces inwardly face each other, said load gauge isinterposed between the central portions of said two rigid plates, asupport is interposed between the longitudinal end portions of said tworigid plates, and a filter is interposed between the transverse endportions of said two rigid plates. This configuration can be applied toa single use of a pressure converter; however, when a plurality ofpressure converters are connected in a longitudinal column to form amulti-stage arrangement so as to measure the distribution of effectiveearth pressure at a plurality of positions, it is possible to provide aslim configuration. As a result, it is possible to provide a pressureconverter which can easily be inserted into a relatively small bore holeand can easily and accurately detect the effective earth pressure.

According to the present invention, the shape of the transversecross-section of the pressure receiving surface of said rigid plate isgenerally semicircular, a thin-walled elastic portion is disposed in thevicinity of the longitudinal end portion of at least one rigid plate,the opposite sides of the pressure receiving surfaces inwardly face eachother, said load gauge is interposed between the central portions ofsaid two rigid plates, and a support and a filter are interposed betweenthe end portions of said two rigid plates. Therefore, when the pressureconverter is inserted into a bore hole the outer circumferential surfaceof the pressure converter uniformly comes in contact with the inner wallof the bore hole. As a result, it is possible to provide a pressureconverter, which senses fluctuation of the ground more a accurately whenit is buried in the ground, thereby more accurately detecting theeffective earth pressure.

According to the present invention, said two rigid plates are coupled inthe longitudinal direction in a multi-stage arrangement, and an outputsignal cable of said load gauge successively passes through an air gapdefined as the result of said load gauge being interposed between saidtwo rigid plates. Accordingly, the signal cable dose not come out fromthe pressure converter, and therefore, the signal cable does notinterfere with the observed earth pressure as it does in theconventional examples nor is it exposed to an environment where it isabraded. As a result, it is possible to provide a pressure converter,which can detect the effective earth pressure distributed in thelongitudinal direction.

Furthermore, according to the present invention, a thin-walled elasticportion is disposed at an end portion of at least one of two rigidplates which receive earth pressure, an optical-fiber strain gauge orstrain gauge is attached to the opposite surface side from the pressurereceiving surface of said thin-willed elastic portion, the oppositesides of the pressure receiving surfaces inwardly face each other, asupport and a filter are interposed between the end portions of said tworigid plates, and said optical-fiber strain gauge or strain gauge, whichdoes not cause output fluctuation even if air pressure and hydraulicpressure are directed via said filter and applied between said two rigidplates, detects a signal corresponding to the effective earth pressure.Accordingly, this pressure converter does not require a separate loadgauge, and an optical-fiber strain gauge or a strain gauge may bedirectly attached to the thin-walled elastic portion, resulting in costreduction. Therefore, it is possible to provide a load converter(=pressure converter) which has the same effects as the presentinvention.

According to the present invention, a filter having slow responsivenessto hydraulic pressure is interposed between the end portions of said tworigid plates which receive hydraulic pressure, a load gauge isinterposed between the central portions of said two rigid plates, afilter having fast responsiveness to hydraulic pressure is disposed sothat it surrounds said two rigid plates, and said load gauge detects asignal corresponding to the dynamic pore water pressure. Therefore, itis possible to provide an inexpensive, small pressure converter, whichdoes not use bellows as does the above-mentioned second conventionalexample. Moreover, the pressure converter has an excellent resolutionand can accurately measure the low dynamic pore water pressure in theground because it is not affected by hydrostatic pressure caused bywater depth and soil pressure. As a result, it is possible to monitorvery small earth pressure fluctuation that occurs in the ground therebycontributing to disaster prediction.

According to the present invention, a thin-walled elastic portion isdisposed at an end portion of at least one of two rigid plates whichreceive earth pressure, an optical-fiber strain gauge or strain gauge isattached to the opposite surface side from the pressure receivingsurface of said thin-walled elastic portion, the opposite sides of thepressure receiving surfaces inwardly face each other, a filter havingslow responsiveness to hydraulic pressure is interposed between the endportions of said two rigid plates, a filter having fast responsivenessto hydraulic pressure is disposed so that it surrounds said two rigidplates, and said optical-fiber strain gauge or strain gauge detects asignal corresponding to the dynamic pore water pressure. Therefore, itis possible to provide an inexpensive, simply configured pressureconverter, which is cable of detecting dynamic pore water pressurewithout being influenced by hydrostatic pressure caused by the depth ofinstallation.

Moreover, according to the present invention, a plurality of supportsareis provided between adjacent pressure converters, and a couplingplate extends between adjacent supports and is mounted to said supportsso as to connect a plurality of pressure converters in a longitudinalcolumn arrangement, thereby distributing loads in the directions otherthan the measurement direction to said supports and said coupling plate.Therefore, a plurality of pressure converters is are fly connected andthe strength in the direction of compression and torsion strength arereinforced, thereby preventing the pressure converter from being damagedwhen it is inserted into a bore hole. Also, it is possible to provide apressure converter, which can detect effective earth pressuredistributed in the longitudinal direction while preventing pressuresother than the pressure in the direction of the measurement from beingapplied to the pressure converter at the time of the measurement.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not as restrictive. The scope of the invention is, therefore,indicated by the appended claims and their combination in whole or inpart rather than by the foregoing description. All changes that comewithin the meaning and range of equivalency of the claims are to beembraced within their scope.

1. A pressure converter, comprising: two rigid plates facing each other,said rigid plates receiving earth pressure, the earth pressure being asum of pore pressure and soil skeleton pressure; a load gauge, whichdoes not cause output fluctuation even if air pressure and hydraulicpressure are directed via a filter and applied between two rigid plates,said gauge being installed in an air gap defined by said two rigidplates; a filter interposed between end portions of said two rigidplates; and said load gauge detecting a signal corresponding to theeffective earth pressure, the effective earth pressure being the soilskeleton pressure.
 2. A pressure converter according to claim 1, whereina thin-walled elastic portion is disposed at an end portion of at leastone of said two rigid plates so that a load in proportion to earthpressure can be transmitted to said load gauge.
 3. A pressure converteraccording to claim 1, wherein the shape of an exterior surface of saidrigid plate is circular; a thin-walled elastic portion is disposed inthe vicinity of the peripheral edge of the opposite surface from thepressure receiving surface of at least one rigid plate; said two rigidplates are disposed so that their opposite surfaces from the pressurereceiving surfaces inwardly face each other; said load gauge isinterposed between the central portions of said two rigid plates; and asupport and said filter are interposed between the peripheral edgeportions of said two rigid plates.
 4. A pressure converter according toclaim 1, wherein the shape of the pressure receiving surface of saidrigid plate is rectangle; a thin-walled elastic portion is disposed inthe vicinity of the longitudinal end portion of the opposite surfacefrom the pressure receiving surface of at least one said rigid plate;the opposite sides of the pressure receiving surfaces inwardly face eachother; said load gauge is interposed between the central portions ofsaid two rigid plates; a support is interposed between the longitudinalend portions of said two rigid plates; and said filter being interposedbetween opposing side end portions of said two rigid plates.
 5. Apressure converter according to claim 1, wherein the shape of thecross-section of the pressure receiving surface of said rigid plate,perpendicular to a long axis of said rigid plate, is generallysemicircular; a thin-walled elastic portion is disposed in the vicinityof the longitudinal end portion of at least one rigid plate; theopposite sides of the pressure receiving surfaces inwardly face eachother; said load gauge is interposed between the central portions ofsaid two rigid plates; a support is interposed between the longitudinalend portions of said two rigid plates; and said filter being interposedbetween opposing side end portions of said two rigid plates.
 6. Apressure converter according to claim 1, wherein said two rigid platesare coupled in the longitudinal direction; and an output signal cable ofsaid load gauge successively passes through said air gap defined as theresult of said load gauge being interposed between said two rigidplates.
 7. A pressure converter, wherein two rigid plates facing eachother, said rigid plates receiving earth pressure, the earth pressurebeing a sum of pore pressure and soil skeleton pressure; a thin-walledelastic portion is disposed at an end portion of at least one of saidtwo rigid plates; an optical-fiber strain gauge or strain gauge isattached to the opposite surface side from the pressure receivingsurface of said thin-walled elastic portion; the opposite sides of thepressure receiving surfaces inwardly face each other; a support and afilter are interposed between the end portions of said two rigid plates;and said optical-fiber strain gauge or strain gauge, which does notcause output fluctuation even if air pressure and hydraulic pressure aredirected via said filter and applied between said two rigid plates,detecting a signal corresponding to the effective earth pressure, theeffective earth pressure being the soil skeleton pressure.
 8. A pressureconverter, wherein a filter having slow responsiveness to hydraulicpressure is interposed between end portions of two rigid plates whichreceive hydraulic pressure; a load gauge is interposed between thecentral portions of said two rigid plates; a filter having fastresponsiveness to hydraulic pressure is disposed so that it surroundssaid two rigid plates; and said load gauge detects a signalcorresponding to the dynamic pore water pressure.
 9. A pressureconverter, wherein a thin-walled elastic portion is disposed at an endportion of at least one of two rigid plates which receive earthpressure; an optical-fiber strain gauge or strain gauge is attached tothe opposite surface side from the pressure receiving surface of saidthin-walled elastic portion; the opposite sides of the pressurereceiving surfaces inwardly face each other; a filter having slowresponsiveness to hydraulic pressure is interposed between the endportions of said two rigid plates; a filter having fast responsivenessto hydraulic pressure is disposed so that it surrounds said two rigidplates; and said optical-fiber strain gauge or strain gauge detects asignal corresponding to the dynamic pore water pressure.
 10. A pressureconverter according to claim 6, further comprising a plurality ofsupports provided between longitudinally coupled rigid plates; and acoupling plate extending between adjacent supports, said coupling platebeing mounted to said supports so as to connect a plurality of pressureconverters in a longitudinal column arrangement.